Engineering the Electronic and Thermal Properties of Two-Dimensional Covalent Organic Frameworks

MA Rahman and S Thakur and PE Hopkins and A Giri, JOURNAL OF PHYSICAL CHEMISTRY C, 127, 11157-11166 (2023).

DOI: 10.1021/acs.jpcc.3c00652

Two-dimensional covalent organic frameworks (2D COFs)are a classof modular polymeric crystals with high porosities and large surfaceareas. Their tunable microstructure (with a wide array of choicesfor the molecular building block) provides the opportunity for theirbottom-up design and potentially tailorable physical properties. Inthis work, through combined density functional theory (DFT) calculationsand molecular dynamics (MD) simulations, we study the influence ofdifferent molecular functional groups and varying porosities on theelectronic and thermal properties of 2D COFs. More specifically, byperforming DFT calculations on 24 different 2D COFs, we demonstratethat one of the main descriptors dictating their band gaps are theirmass densities or network porosities. Furthermore, we also find thatspecific functional groups forming the nodes can lead to larger localizationof charge densities resulting in wider band gaps. By performing MDsimulations to investigate their thermal properties, we show that(similar to their electronic properties) mass density is also oneof the main factors dictating heat conduction, where higher densitiesare associated with relatively higher thermal conductivities alongthe 2D sheets. Our spectral energy density calculations provide insightsinto the highly anharmonic nature of these materials. We find thatincreasing porosities lead to larger anharmonic interactions and thusreduced thermal conductivities in these materials. Similar to theirelectronic band gaps, the nodes forming the 2D COFs also have a significantcontribution in dictating their thermal conductivities with biggernodes (accompanied by higher densities) generally resulting in relativelyhigher thermal conductivities in 2D COFs. Taken together, the resultingchanges in the electronic and thermal properties from variations inthe building blocks in 2D COFs lend insights into fundamental changesin the microscopic thermodynamics that arise from systematically changingtheir molecular structure. Therefore, our study provides a blueprintfor the strategic syntheses of 2D COFs with "user- defined"electronic and thermal properties that will ultimately aide in theirincorporation into various applications.

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